The technical field of this invention is spectrometry, e.g., ion mobility spectrometry and the like, and in particular to methods and reagents that can enhance the ionization of low volatility analytes.
The threats posed by concealed explosives and the intentional release of toxic chemicals continue to drive improvements in ways to detect these threats as a means to protect the public. One technique used to aid in this mission involves identification of the threat molecule by first ionizing it, and then detecting whether the threat molecule (analyte) is present. Ion mobility spectrometry (IMS), differential mobility spectrometry (DMS), field asymmetric ion mobility spectrometry (FAIMS), and mass spectrometry (MS) are all methods used to identify molecules that first require ionization. Given the importance of these techniques to public safety, considerable effort has been devoted to develop the best means to collect the sample from the environment, present the sample to the instrument, and to also ionize it efficiently and, if possible, selectively in order to provide the greatest detection capability. In almost all instances, the ionization is achieved at ambient-pressure using a technique called ambient-pressure ionization (API) (also sometimes called atmospheric-pressure chemical ionization). Because the ionization occurs in the gas phase through ion-molecule collisions, it is usually essential that the analyte is first vaporized to be present in the gas phase.
However, many of the explosive and chemical threats have low vapor pressure and exist as traces of solid particulates or thin films on surfaces, and thus the most common way to collect the sample requires a swipe or swab substrate which provides a physical mechanism to both collect and preconcentrate the sample off the contaminated surface for subsequent presentation to the ionization space of the detection instrument. The substrate media, or “swipe”, containing the collected sample can be used to present the sample to the ionization instrument in one of two ways. In the first method, the swipe can be extracted using a solvent to selectivity dissolve the collected analyte into a solvent liquid, which is then presented to the ionization space of the instrument via a process called electrospray ionization. (See U.S. Pat. Nos. 8,513,596; 5,157,260; and 5,756,994, for examples of this approach). This requires extraction, dissolution, and injection steps and, although effective, is not practical in field settings. The alternative, and currently preferred, method is to heat the swipe to desorb the target chemical into the vapor phase for subsequent ionization and detection.
In a typical API system, a swipe or swab substrate is positioned in a thermal desorber located on the inlet side of the detection system. Thermal heating of the solid analyte particles on the swipe induces a solid-to-vapor phase transition and releases the analyte molecules as a vapor, usually guided into the sensor inlet by a carrier gas, and ionization occurs in the vapor phase. Properties of commercially-available swipe media have been optimized over the years for increased efficiency of particle collection from surfaces (mechanical or electrostatic), efficient transfer and release of analyte into the chemical sensor, thermal stability, and low chemical background of the substrate. Prior art exists in the patent literature on different embodiments of sampling swipes (e.g., US2006-0192098; EP1844189 and WO2007-066240). See also, commonly-owned U.S. patent application Ser. No. 13/832,905 filed entitled “Reagent Impregnated Swipe for Thermal Desorption Release and Chemical Detection with Ambient Ionization Techniques,” which discloses reagents that are chemically embedded in the swipe material for interaction with the analyte.
One area where such an approach may be beneficial is for detection of materials such as the sodium and potassium salts of chlorate and perchlorate, which do not have appreciable vapor pressures to allow for sensitive detection at desorption temperatures less than 250° C. Reagents that convert these compounds to more volatile compounds at temperatures less than 250° C. can thus provide one means to improve performance of API-based detection systems and/or allow for operation at lower temperatures.